(1) Field of the Invention
The present invention relates generally to rims for wheels for human powered vehicles such as bicycles, wheel chairs and the like, and especially to light weight high performance wheels with light weight rims having deep sections with thin walls and methods for producing them.
(2) Description of Related Art
A great deal of effort has been directed to reducing the weight of human powered wheel rims, especially bicycle rims. Bicycle wheel rims are typically produced by extruding aluminum alloy or the like to produce a linear extrusion having a cross section corresponding with the finished rim, rolling the extrusion into a circular shape, cutting the extrusion to length and connecting the free ends with pins, plugs or welds. In the case of deep V-shaped, aero rims, the rolling of the linear extrusion imposes enormous forces on the extrusion. These forces far exceed the forces that the finished rim must endure in use on a bicycle. Consequently, the thickness of the rim walls tend to be greater than the thickness required for service as a bicycle wheel rim. Typical rim sidewall thicknesses for a bicycle wheel rim tend to be at least 1 mm for V-shaped rims, and thicker for deep sections. The thickness of the sidewalls is required to prevent the sidewalls from buckling during the bending of the linear extrusion in the rolling operation.
U.S. patent application publication No. US 2004/0163255, published Aug. 26, 2004, discloses a method for manufacturing light weight thin walled deep V-section rims by first extruding a rim section with sidewalls much thicker and heavier than the desired rim sidewalls. The extrusion can be rolled to form the rim without buckling of the sidewalls and then the excess material can be removed from the sidewalls, for example by sanding, using flap wheels. This method works but has some disadvantages, such as non-precise control of the sidewall thickness and expense of the process.
European Patent No. 0715001 discloses a technique for producing a bicycle wheel rim involving the production of a linear extrusion, rolling the extrusion, cutting the extrusion to length and joining the free ends to produce a rim blank. The thickness of the walls of the rim blank exceed the desired wall thickness for a finished rim and the rim blank is immersed in a caustic chemical to remove metal from the rim until a desired wall thickness is achieved. The rim blank is then rinsed, immersed in a neutralizing bath, rinsed again and dried. This method reduces the thickness of the rim blank walls generally uniformly for small amounts of reduction.
U.S. Pat. No. 5,651,591 discloses a bicycle wheel rim having a lower bridge or rim floor having a thickness of about 0.7 mm. According to the patent, the rim blank is extruded with side walls (wings) and a lower flange (lower bridge) that are thicker than the final dimensions. After the extrusion is rolled, cut and joined, the rim blank is machined by chemical machining, to reduce uniformly the thickness of the wings and bridges, or mechanically machined to reduce the thickness of the lower bridge. The mechanical machining is carried out by milling, turning or grinding.
European patent No. 0579525 discloses a rim production technique according to which an extrusion is rolled and cut to length and the ends are joined to produce a rim blank. The rim blank is then machined to form two opposed braking surfaces by machining material from the two opposed flanks. The machining is carried out by turning or by using a cutter.
Deep V-section rims or aero rims pose unique fabrication challenges. When the linear extrusion is bent in the rolling operation, the relatively deep and thin sidewalls must not buckle. Consequently, deep V-section rims have side walls that are thick enough to survive the rolling operation without buckling, which means that they are substantially thicker than they need to be to stand up to the stresses that they see in service. Thus, there is a weight penalty associated with deep V-section aero rims produced in this way and, because the weight is at or near the outer portion of the wheel, it degrades the rotational inertia properties of the wheel.